Tissue supporting devices

ABSTRACT

A new multiple component stent arrangement which allows for initial self-expansion and subsequent deformation to a final enlarged size.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.08/737,492, filed Mar. 19, 1997, now U.S. Pat. No. 6,582,461, which is a§371 National Stage Application of PCT/US95/06228 filed May 18, 1995,which is a Continuation-in-Part of U.S. application Ser. No. 08/246,320filed May 19, 1994, now abandoned. All of the applications referred toin this paragraph are incorporated herein in their entirety byreference.

BACKGROUND OF THE INVENTION

This invention relates to tissue supporting devices in general and mostparticularly to vascular stents for placement in blood vessels. Aprimary feature of the devices of this invention is that they areexpandable within the body.

In the past, such devices have been provided for implantation withinbody passageways. These devices have been characterized by the abilityto be enlarged radially, often having been introduced into the desiredposition in the body as by percutaneous techniques or surgicaltechniques.

These devices are either expanded mechanically, such as by expansion ofa balloon positioned inside the device, or are capable of releasingstored energy to self-expand themselves within the body.

The materials which have been used to make up these devices haveincluded ordinary metals, shape memory alloys, various plastics, bothbiodegradable and not, and the like.

This invention is concerned with the use of these materials in a newmultiple component arrangement which allows for initial self-expansionand subsequent deformation to a final enlarged diameter in the body.

Balloon expandable stents do not always expand uniformly around theircircumference. As a result, healing may not take place in a consistentmanner. If the stent is coated or covered, non-uniform expansion maytear the covering or coating. Additionally, long stents of this type mayrequire long balloons which can be difficult to handle, difficult tosize, and may not offer ideal performance in tortuous passages in bloodvessels and the like.

Thus, when addressing such issues, self-expandable stents have beenthought to be generally more desirable. Unfortunately, one cannotcontrol the degree of expansion and hence the degree of embedment in thevessel wall. It has been determined that a stent must be embedded tosome degree to be clinically satisfactory.

The stents of the present invention provide the best features of both ofthese types of stents without their drawbacks.

SUMMARY OF THE INVENTION

The tissue supporting devices of this invention are generallycylindrical or tubular in overall shape and of such a configuration asto allow radial expansion for enlargement. They are often referred toherein in the general sense as “stents”. Furthermore, the devices arecomprised of at least one component, element, constituent or portionwhich exhibits a tendency to self-expand the device to an expanded sizeand at least one other component, element, constituent or portion whichis deformable so as to allow an external force, such as a balloonpositioned within the body of the device, to further expand it to afinal, larger desired expanded size. The terms “component”, “element”,“constituent” and “portion” are often referred to herein collectively as“portion”.

Preferably, the devices of the invention are made of metal and mostpreferably of shape memory alloys.

In one embodiment, a first portion is a resilient spring-like metal forself-expansion and a second portion is a deformable metal for finalsizing. In a more preferred shape memory embodiment, a first portion isa self-expanding austenitic one and a second is a martensitic onecapable of deformation. In the case of shape memory embodiments the“portions” may be discrete or merely different phases of an alloy.

The most preferred embodiment of the invention is a stent, preferably ofshape memory alloy. The most preferred shape memory alloy is Ni—Ti,although any of the other known shape memory alloys may be used as well.Such other alloys include: Au—Cd, Cu—Zn, In—Ti, Cu—Zn—Al, Ti—Nb,Au—Cu—Zn, Cu—Zn—Sn, Cu—Zn—Si, Cu—Al—Ni, Ag—Cd, Cu—Sn, Cu—Zn—Ga, Ni—Al,Fe—Pt, U—Nb, Ti—Pd—Ni, Fe—Mn—Si, and the like. These alloys may also bedoped with small amounts of other elements for various propertymodifications as may be desired and as is known in the art.

The invention will be specifically described hereinbelow with referenceto stents, a preferred embodiment of the invention although it isbroadly applicable to tissue support devices in general.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a braided stent according to one embodiment of this invention.

FIG. 2 is a graph showing the martensitic/austenitic temperaturetransformation curve and the superelastic area of a shape memory alloy.

FIG. 3 is an end view of a layered stent having two discrete componentsaccording to one aspect of this invention.

FIGS. 4 a and 4 b are graphs showing the martensitic/austenitictemperature transformation curves of the layers in the stent of FIG. 3.

FIGS. 5 a and 5 b are views of another embodiment of the inventioncomprised of alternating rings of shape memory alloy.

FIG. 6 is a showing of a stent fragment of a braided version of a shapememory stent of this invention.

FIG. 7 is a graph showing a temperature window for a shape memory alloyto be used in yet another stent version of this invention.

FIG. 7 a is a graph showing expansion of a stent with temperature.

FIG. 7 b is a graph of the same type, the stent having been cold-worked.

FIG. 7 c is a graph of the same type, the stent having had pseudoelasticprestraining.

FIG. 7 d is a graph of the same type, the stent having amnesiainducement.

FIGS. 8-11 show various expandable configurations (closed and open)illustrated in fragment which may be used in the stents of thisinvention. FIGS. 9 a and 9 b show a preferred embodiment of anarticulated stent.

FIG. 12 shows another version of an expandable stent of the invention.

FIG. 13 shows yet another version of a stent which may be used with theinvention.

FIG. 14 is a schematic showing of a braided stent made up of a pluralityof strands.

FIG. 15 is a detail of a single strand from the stent of FIG. 14 showingthat the strand is made up of a plurality of wires of two differenttypes.

FIG. 16 is a cross-sectional view taken along line 16-16 of FIG. 15showing the two different types of wire.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of this invention are described below withparticular reference to the accompanying drawing Figures.

Referring first to the embodiment shown in FIG. 1, a stent 10 is showncomprised of braided or interwoven metal strands 12 and 14. Strands 12are of a resilient spring-like metal such as spring steel, Elgiloy forexample. Preferably, strands 12 are spirally extending in the samedirection, spiraling to the right as seen in FIG. 1. Strands 14 are of adeformable or annealed metal such as stainless steel and are preferablyspiraled in the opposite direction as strands 12, as shown in FIG. 1.

Given such a stent construction of two components i.e., strands 12 and14, it can be seen that stent 10 may be readily loaded on a catheter asby placing it over an uninflated balloon on a balloon catheter andcompressing it tightly around the balloon and then placing a sheath overthe stent to hold it in place during the transluminal placementprocedure. Once in place, the sheath is removed, for example slid back,to expose the stent, allowing it to self-expand by force of theresilient strands 12 to substantially assume a self-expanded shape/size.Some self-expansion may be restrained if held back by strands 14. Tofinally adjust the size of the stent, the balloon may be expanded byinflation from within the stent to exert an outward radial force on thestent and further enlarge it by stretching and deforming the deformablemetal of strands 14. This may be aided by building into strands 14, aseries of readily deformable structures or means such as bends or kinks16 as shown in FIG. 1. It can be seen that a permanent adjustable sizebeyond the self-expanded size may be obtained with this embodiment. Itis to be noted that many configurations other than braided may bereadily devised to take advantage of this two component concept,including various of the subsequent configurations describedhereinbelow. Also, it should be noted that, although not preferred, thestent may be initially deployed without a balloon; the balloon followingon a separate catheter.

Referring now to subsequent features, other preferred embodiments of theinvention will be described which make use of shape memory alloys andsome of their unique properties, primarily their special types ofdeformation i.e., shape memory deformation in martensite and/orsuperelastic deformation in austenite.

The term “superelasticity” is used to describe the property of certainshape memory alloys to return to their original shape upon unloadingafter a substantially deformation while in their austenitic state.Superelastic alloys can be strained while in their austenitic state morethan ordinary spring materials without being plastically deformed. Thisunusually large elasticity in the austenitic state is also called“pseudoelasticity”, because the mechanism is nonconventional in nature,or is also sometimes referred to as “transformational superelasticity”because it is caused by a stress induced phase transformation. Alloysthat show superelasticity also undergo a thermoelastic martensitictransformation which is also the prerequisite for the shape memoryeffect. Superelasticity and shape memory effects are therefore closelyrelated. Superelasticity can even be considered part of the shape memoryeffect.

The shape memory and superelasticity effects are particularly pronouncedin Ni—Ti alloys. This application will therefore focus on these alloysas the preferred shape memory alloys. The shape memory effect in Ni—Tialloys has been described many times and is well known.

In near-equiatomic Ni—Ti alloys, martensite forms on cooling from thebody centered cubic high temperature phase, termed austenite, by a sheartype of process. This martensitic phase is heavily twinned. In theabsence of any externally applied force transformation takes place withalmost no external macroscopic shape change. The martensite can beeasily deformed by a “flipping over” type of shear until a singleorientation is achieved. This process is also called “detwinning”.

If a deformed martensite is now heated, it reverts to austenite. Thecrystallographic restrictions are such that it transforms back to theinitial orientation thereby restoring the original shape. Thus, if astraight piece of wire in the austenitic condition is cooled to formmartensite it remains straight. If it is now deformed by bending, thetwinned martensite is converted to deformed martensite. On heating, thetransformation back to austenite occurs and the bent wire becomesstraight again. This process illustrates the shape memory deformationreferred to above.

The transformation from austenite to martensite and the reversetransformation from martensite to austenite do not take place at thesame temperature. A plot of the volume fraction of austenite as afunction of temperature provides a curve of the type shown schematicallyin FIG. 2. The complete transformation cycle is characterized by thefollowing temperatures: austenite start temperature (A_(s)), austenitefinish temperature (A_(f)), both of which are involved in the first part(1) of an increasing temperature cycle and martensite start temperature(M_(s)) and martensite finish temperature (M_(f)), both of which areinvolved in the second part (2) of a decreasing temperature cycle.

FIG. 2 represents the transformation cycle without applied stress.However, if a stress is applied in the temperature range between A_(s)and M_(d), martensite can be stress-induced. Stress induced martensiteis deformed by detwinning as described above. Less energy is needed tostress induce and deform martensite than to deform the austenite byconventional mechanisms. Up to about 8% strain can be accommodated bythis process (single crystals of specific alloys can show as much asabout 25% pseudoelastic strain in certain directions). As austenite isthe thermodynamically stable phase at temperatures between A_(s) andM_(d) under no-load conditions, the material springs back into itsoriginal shape when the stress is no longer applied.

It becomes increasingly difficult to stress-induce martensite atincreasing temperatures above A_(f). Eventually, it is easier to deformthe material by conventional mechanisms (movement of the dislocation,slip) than by inducing and deforming martensite. The temperature atwhich martensite can no longer be stress-induced is called M_(d). AboveM_(d), Ni—Ti alloys are deformed like ordinary materials by slipping.

Additional information regarding shape memory alloys is found in thefollowing references, all of which are incorporated fully herein byreference:

-   -   “Super Elastic Nickel-Titanium Wires” by Dieter Stöckel and        Weikang Yu of Raychem Corporation, Menlo Park, Calif., copy        received November 1992;    -   Metals Handbook, Tenth Edition, Vol. 2, Properties and        Selection: Nonferrous Alloys and Special-Purpose Materials,        “Shape Memory Alloys” by Hodgson, Wu and Biermann, pp. 897-902;        and,    -   In Press, Titanium Handbook, ASM (1994), Section entitled        “Structure and Properties of Ti—Ni Alloys by T. W. Duerig        and A. R. Pelton.

Since the most preferred shape memory alloy is Ni—Ti, the martensiticstate of this alloy may be used to advantage in the two componentconcept of this invention.

For example, with reference to FIG. 3, a layered construction may beprovided in a stent 30 (shown in end view) which is generally a hollowcylindrical or tubular body in shape but which may be formed in a widevariety of specific configurations or patterns to foster radialexpansion of the body as exemplified in FIGS. 1, 5, 6 and in subsequentFIGS. 8-11.

Stent 30 is comprised of at least two layers 32 and 34, one of which 32is a Ni—Ti alloy (50.8 atomic wt. % Ni, balance Ti, transitiontemperature of A_(f)=0° C.) and normally in the austenitic state, theother of which 34 is a Ni—Ti (49.4 atomic wt. % Ni, balance Ti,transition temperature A_(f)=60° C.) and normally in the martensiticstate. Preferably, the inner layer is 32 and the outer layer is 34.However, this may be reversed and also a plurality of layers,alternating or otherwise, may be utilized in this particular embodiment.

Stent 30 is made to a fabricated size and shape (parent shape) whichprovides austenitic layer 32 its parent shape and size i.e., itssuperelastic high temperature shape and size. Obviously, in its asfabricated condition, the Ni—Ti alloy of austenitic layer 32 is selectedso as to have a transition temperature range between its austenitic andmartensitic states which is lower than body temperature as to ensurethat in the body and at body temperatures the austenitic state willalways prevail.

On the other hand, martensitic layer 34 is of a Ni—Ti alloy having atransition temperature range significantly greater than body temperatureso as to ensure that under body conditions the martensitic state willalways prevail and the alloy will never transform to austenite in stentuse. This is shown in the graphs of FIGS. 4 a and 4 b which demonstratethe relative transition temperatures of layers 32 and 34, respectivelyfor purposes of this invention. It can be seen from these graphs thatthe normal condition of layer 32 (FIG. 4 a) at body temperatures andhigher is the austenitic state while the normal condition of layer 34(FIG. 4 b) at body temperatures is martensitic.

To manufacture the layered construction, one may make the austeniticportion with any standard metallurgical technique and vapor deposit themartensitic portion on its surface. Other manufacturing techniques suchas diffusion bonding, welding, ion beam deposition, and various otherswill be apparent to those familiar with this art.

Such a stent may be compressed or constrained (deformed to a smalldiameter) onto a balloon catheter as described for the previousembodiment and captured within a sheath. During the constrainment,austenitic layer 32 may stress induce to a martensitic state. In thealternative, the stent may be cooled below the transition temperature oflayer 32 to facilitate its deformation and constrainment. Martensiticlayer 34 merely undergoes deformation. Thus the stent may be “loaded”onto a balloon catheter. However, with temperature changes occurring upto body temperature, layer 32 will remain martensite until theconstraint is removed. When released in place in the body, stent 30 willexpand to a percentage of its self-expanded size and shape due to thetransformation of layer 32 from martensite to austenite at which pointthe balloon may be used to radially expand the stent to a greaterpermanent diameter by deforming martensitic layer 34. On the other hand,initial deployment can take place without a balloon which may beseparately inserted after deployment.

The two component concept of the invention in the layered embodiment ofFIG. 3 requires both the martensitic and austenitic phasecharacteristics of shape memory alloy(s) in the two discrete components32 and 34.

Preferably, the stent is fabricated in such a way that the austeniticlayer 32 tends to self-expand stent 30 to a predetermined fabricateddiameter (parent shape). The martensitic layer 34 tends to hold backthis self-expansion, preventing full expansion. For example, the stentmay only be able to self-expand to 75% of its full possible diameter (asdetermined by the austenitic layer). Therefore, expansion beyond 75% isaccomplished by an applied external force, as by the balloon inside thestent. It is suggested that the stent not be expanded beyond its normalfabricated diameter for the austenitic layer 32 will have the tendencyof making the stent diameter smaller as it tries to recover itsfabricated diameter (parent shape). If the stent is subjected to atemperature above body temperature and above the transition temperatureof the martensitic layer (which is clinically unlikely), the stent willself-expand to the fabricated diameter only. Depending on design sizethere are thus provided permanent stents capable of fulfilling anyneeded range of sizes with an adjustable sizing capability.

As is known in the art, the desired properties of the shape memoryalloys required for use in this invention may be obtained by alloycomposition and working and heat treatment of the alloys, in variouscombinations or singly.

Manufacturing techniques influence the phase characteristics of thematerial. Alloy composition, work history, and heat treatment allinfluence the final characteristics. At a specific operatingtemperature, say body temperature, the austenite phase material willhave a transition temperature below body temperature (i.e., A_(f)=0°C.). The material is capable of taking high strains and recovering afterthe load is released. The martensite phase material will have a highertransition temperature than body temperature (i.e., A_(f)=60° C.), andis characteristically soft and pliable and retains the deformed shapeafter load removal. This martensite deformation is caused by detwinning,not the typical plastic deformation, or yielding, of crystal slip.

With reference to FIGS. 5 and 6, other stent constructions are shownwhich are similar to the layered version of FIG. 3 in so far asutilization of the two component concept of this invention is concerned.

FIGS. 5 a and 5 b shows a stent 50 made up of alternating expandablerings 52 and 54 of austenitic and martensitic alloys, respectively,analogous to layers 32 and 34 of the FIG. 3 embodiment. Rings 52 and 54for example are interconnected by strut members 56 which may be of anymaterial capable of rigidly holding the rings together. Otherinterconnector means may be used. As can be seen in FIG. 5 b, theplacement of strut members 56 does not require them to take part in theradial expansion of the stent and they can therefore be of a relativelyordinary material such as stainless steel.

Referring now to FIG. 6, a braided or interwoven construction is shownsimilar in construction to that of the embodiment of FIG. 1. In thisembodiment, strands 62 extending to the right in FIG. 6 are an alloy inthe austenitic state whereas strands 64 extending to the left in FIG. 6are an alloy in the martensitic state.

Referring now to the graph of FIG. 7, it is demonstrated that the twocomponent concept of the invention may be embodied in two phases, i.e.,components of a single shape memory alloy and need not be in the form oftwo discrete components such as layers, members, wires, etc. In thegraph of FIG. 7, it can be seen that an alloy composition can beselected such that it has a phase transition temperature window thatbounds the proposed operating temperatures of the stent, such as thenormal body temperature range. Within this transitional window or zone,the material undergoes the phase transition and is effectivelycompositionally comprised of a ratio of austenitic to martensitic phasedepending on the temperature of the stent. This ratio should be selectedso as to provide sufficient radial force from the austenite phase whilestill allowing for further expansion of the martensite phase with amechanical expansion means such as a balloon. Selecting body temperatureas the operating temperature, a Ni—Ti alloy of about 50/50 atomic wt. %composition (range about 49/51%) will provide an acceptable “window” forthis embodiment, the two components are the austenite and martensitephases of the nitinol.

The method of making a stent may be described as follows. Age the shapememory material (Ni Ti) until body temperature falls somewhere withinthe transformation window. Therefore the stent will not fully recover toits high temperature shape at body temperature. An example of thistechnique is described below.

A stent of tubular 50.8% Ni balance Ti was prepared having a 1.5 mmdiameter. It was substantially all austenite at room temperature, theA_(f) being about 15-20° C. and therefore being superelastic at roomtemperature. The stent was cooled to below room temperature to formsubstantially all martensite and mechanically expanded to 4.7 mm indiameter. It was maintained at the 4.7 mm in diameter and heat treatedat 500° C. for 30 minutes and water quenched. Finally, it was againcooled to below room temperature to form substantially all martensiteand compressed to a diameter of 1.5 mm. After deployment and at bodytemperature the stent has a diameter of 3.5 mm. At about 70% of fullexpansion, i.e., about 40° C. it had a diameter of 4.5 mm and at 42° C.it had a fully expanded diameter of 4.7 mm.

This method works fairly well, but due to the slope of the temperatureversus diameter plot being extremely vertical at body temperature, asmall change in body temperature, or manufacturing control, can have alarge impact on the actual self expansion diameter. As can be seen fromFIG. 7, the slope of the line between A_(f) and A_(s) is rather steepwith small changes in temperature leading to large changes in percentaustenite and consequently large changes in diameter of a stent made ofsuch an alloy. FIG. 7 a shows a temperature versus diameter plot. Threemethods of modifying the slope of the line on the temperature versusdiameter graph are cold work, pseudoelastic prestraining, and amnesiainducement, illustrated in FIGS. 7 b, 7 c and 7 d, respectively.

Cold Work

Residual cold work in nitinol draws out or masks the point of A_(f) onthe diameter versus the temperature curve. This is seen by the sluggishincrease in diameter as temperature increases in the last 20-30% ofrecover. By utilizing the effects of cold work, the effects oftemperature change on diameter can be reduced in the last 20 to 30% ofstent expansion. Shown in FIG. 7 b is an example of a temperature versusdiameter plot for a cold worked part. FIG. 7 a above shows an example ofa part without cold work.

Pseudoelastic Prestraining

Utilizing the effects of pseudoelastic prestraining (S. Eucken and T. W.Duerig, ACTA Metal, Vol. 37, No. 8, pp 2245-2252, 1989) one can createtwo distinct plateaus in the stress-strain behavior. This difference instress strain behaviors can be directly linked to two distinct A_(f)temperatures for the two plateaus. By placing the transition between thetwo plateaus at the transition from self expanding to balloon expanding,i.e., 70%, one can control the characteristics of the stent at bodytemperature. The goal would be to place the A_(f) temperature for thefirst plateau (from maximum compression to 70% expansion) below bodytemperature such that the stent has self expanding characteristics. TheA_(f) temperature for the second plateau would be above body temperaturesuch that there is no additional self expansion in this region (70 to100% expansion) a mechanical device, like a balloon, can then be used tocustom size the stent between 70% and 100% of the high temperatureshape. Results of such a technique is shown in FIG. 7 c.

Amnesia Inducement

One of the characteristics of nitinol is cycle amnesia. This was alsodiscussed about in the article referred to immediately above. As nitinolis cycled from its heat set shape as shown in FIG. 7 d, there is anincrease in the amount of amnesia to recover to the heat set shape witheach cycle. As long as this amnesia is not caused by permanent plasticdeformation, the amnesia can be removed by heating the part above M_(d).This shows there is martensite left in the part after cycling which ispreventing full recovery in the austenite phase (just above A_(f)). Thispresence of non recoverable martensite (below M_(d)) is what may be usedfor the balloon expansion region of the stent.

FIGS. 8-11 represent examples of various expandable configurations(a=closed, b=expanded) which may be incorporated into the devices ofthis invention. The version shown in FIGS. 10 a and 10 b may be modifiedas shown in FIGS. 10 c and 10 d (closed and open, respectively) byomitting portions (indicated at 100 in FIGS. 10 c and 10 d) as to renderthe stent flexible for articulation. This may be done to other of thestructures as well to improve flexibility.

Yet another version of a device incorporating the two component conceptof the invention is shown in FIG. 12. In this embodiment, a fragment ofa stent 110 is shown. The stent includes a self-expanding component 112and a deformable, external force expandable component 114. Selfexpanding component 112 may be resilient spring-like metal such astainless steel or it may preferably be a shape memory alloy in theaustenitic state. Component 114 may be any deformable metal or the likesuch as annealed stainless steel or preferably a shape memory alloy inthe martensitic state. The two components may simply be mechanically,welded or bonded together. Functions and operations are as describedhereinabove.

Referring to FIG. 13 a version analogous to the embodiment of FIG. 12 isshown in which the two component concept is again embodied as differentzones or portions of a single metal material.

As shown in FIG. 13, a stent 120 (fragment showing) is of aself-expanding component 122 and a deformable component 124, both ofwhich may be a single metal as spring steel or austenitic Ni—Ti whichhas been appropriately treated with respect to component 124 as bylocalized heat treatment or the like to alter the characteristics of thematerial of the 122 component so as to render it deformable ormartensitic, depending on whether it is merely resilient or isaustenitic. Again, function and operation are the same as with otherembodiments.

Referring now to FIGS. 14-16, a multi-strand braided stent is shown inFIG. 15. Each strand 150 in the stent is a micro-cable. That is, eachstrand is made up of a plurality of wires 152 and 154 as is seen inFIGS. 15 and 16. Each of the wires 152 and 154 consists of two differentnitinol alloys as seen best in FIG. 16, or one nitinol and one ordinarymetal such as stainless steel, platinum or tantalum. The latter twowould provide enhanced radiopacity. One nitinol alloy wire 154 has anaustenitic finish (A_(f)) temperature less than body temperature. Theother wire 152 could be nitinol having an A_(s) (austenitic start)greater than body temperature. Also, it could be an ordinary metal.Additionally, one or more of the strands may be of a biodegradablematerial such as a plastic or may be of a material including anabsorbable drug.

Since the two alloys are stranded into micro-cable one does not have toengage in selective, discrete heat treating methods to produce bothshape memory and martensitic effects.

Radiopaque portions or coatings may be included on any parts of thesestents as is known in the prior art.

While this invention may be embodied in many different forms, there aredescribed in detail herein specific preferred embodiments of theinvention. This description is an exemplification of the principles ofthe invention and is not intended to limit the invention to theparticular embodiments illustrated.

The above Examples and disclosure are intended to be illustrative andnot exhaustive. These examples and description will suggest manyvariations and alternatives to one of ordinary skill in this art. Allthese alternatives and variations are intended to be included within thescope of the attached claims. Those familiar with the art may recognizeother equivalents to the specific embodiments described herein whichequivalents are also intended to be encompassed by the claims attachedhereto.

1. A stent comprising: a plurality of annular elements, each annularelement having a compressed state and an expanded state, wherein eachannular element has a longitudinal dimension which is smaller in theradially expanded state than in the compressed state; and connectingmembers connecting adjacent annular elements; wherein the annularelements and connecting members are made of Nitinol, with eachconnecting member preset with an elasticity which causes the connectingmember to elongate longitudinally when the annular elements are in theirexpanded state to compensate for the smaller longitudinal dimension ofthe annular elements in the expanded state.
 2. The stent of claim 1,wherein each annular element comprises a plurality of alternating strutsand apices connected to each other to form a substantially annularconfiguration.
 3. The stent of claim 2, wherein the connecting membersare connected to the apices of the adjacent annular members.
 4. Thestent of claim 2, wherein the plurality of struts comprises left andright struts, with each pair of left and right struts connected to eachother at an apex.
 5. The stent of claim 2, wherein each strut has alongitudinal dimensional which is smaller when the annular elements arein the expanded state than in the compressed state.
 6. The stent ofclaim 2, wherein each strut has a longitudinal dimensional which islarger when the annular elements are in the compressed state than in theexpanded state.
 7. The stent of claim 2, wherein at least one of theannular elements is closed such that the plurality of alternating strutsand apices are connected to each other to form a closed annular element.8. The stent of claim 1, wherein at least one of connecting member has aplurality of alternating segments.
 9. The stent of claim 8, wherein theat least one connecting member has a plurality of alternating and angledstraight segments.
 10. The stent of claim 1, wherein each connectingmember has a larger longitudinal dimension when each annular element isin the expanded state than in the compressed state to compensate for thesmaller longitudinal dimension of the annular element in the expandedstate.
 11. The stent of claim 1, wherein each connecting member has asmaller longitudinal dimension when each annular element is in thecompressed state than in the expanded state to compensate for the largerlongitudinal dimension of the annular element in the compressed state.12. The stent of claim 1, wherein the annular elements and connectingmembers define an alternating longitudinal pattern of annular elementsand connecting members.